A transition from linear to circular polarization in pulsar radio emission

At present three pulsars are known which clearly show a strongly increasing degree of circular polarization with frequency. As this is accompanied by a smooth decrease of linear polarization, we investigate, if this observati on can be explained through a propagation mode transition from linear to ci rcular polarization. Using a previously published model we find that the ra te of change of the polarization types with frequency is well consistent wi th the theory. The small number of only three objects does of course not allow to draw a g eneral conclusion for the whole sample of pulsars. However we show that the very unusual behavior of these three objects can be well explained with th is model and we discuss, why only so few such objects are presently known.